This invention relates to an apparatus for suppressing and masking infrared emissions from engine exhaust ducts.
The successful flight of high altitude military aircraft is dependent, in part, upon the ability of the aircraft to remain undetected during flight. Unfortunately, the gas stream discharged from the gas turbine engine is a high temperature gas mixture. Because of the high temperatures of the exhaust gases, and the hot metal walls directly in contact with the hot gases, turbine engines emit high levels of infrared energy. Hence, against the highly sophisticated infrared sensors developed during recent years, particularly for military use, an aircraft powered by a gas turbine engine is at a particular disadvantage.
Infrared emissions from gas turbine engines have been reduced by use of a variety of infrared radiation (IR) suppressor designs. Each type of design endeavors to provide a combination of aerodynamics, heat transfer, and geometry which will result in an effective IR suppressor for the least suppressor weight and horsepower effects on a turbine engine. Unfortunately existing suppressors have utilized geometries which have resulted in inherent aerodynamic and heat transfer problems. These geometries have required complex suppressor designs with highly undesirable effects on the engine horsepower.
One of these types of geometries utilizes a concentric center body within an annular duct. This suppressor geometry is referred to as a plug or center body suppressor and exemplified by U.S. Pat. Nos. 4,214,441, 4,044,555, 3,970,252 and the like. The plug suppressors are difficult to analyze aerodynamically and require aerodynamically shaped struts to position and support the center body. These struts exhibit larger temperature gradients and develop high thermal stresses at strut attachment points, presenting severe structural problems. The plug suppressor is difficult to fabricate and results in a heavy suppressor.
Another suppressor geometry being used is the dog-leg or bend-duct type exemplified by U.S. Pat. Nos. 4,002,024, 3,921,906, 4,018,046 and others. The bend-duct suppressor presents inherent aerodynamic and heat transfer problems. The wall at the bend results in either an overly long suppressor or a complicated multi-bend-duct suppressor.
A third geometry type is the radial or horizontal wavy vane exemplified by U.S. Pat. Nos. 4,095,417, 4,007,587, and 4,215,537. The geometry involved is difficult to analyze, design, and fabricate and results in large reductions in engine horsepower and difficult heat transfer problems.
A study of prior art IR suppressors leads to the conclusion that infrared radiation suppression equipment for exhaust gas systems of aircraft require a blend of aerodynamics, heat transfer, and geometry in order to be highly effective in providing the desired infrared suppression levels for the least suppression system weight and engine horsepower. The correct blend of these three technologies has been difficult to achieve. The concentric plug, the bent or curved duct, and the wavy vane geometrics have inherent aerodynamic and heat transfer shortcomings which require complicated and heavy design concepts. Moreover, they require, a significant percentage of engine shaft horsepower. These designs, except for the wavy vane design also require that cooling air be supplied by either engine bleed air operated ejectors, auxiliary blowers, forward flight ram air, or main rotor downwash.
It can be seen that there is still room for improvement in the field of infrared suppressors for turbine engine exhaust ducts. Existing designs show that aerodynamic problems are difficult to analyze and that geometrical parameters are very critical, and nearly impossible to maintain. The invention herein provides the best blend of simple aerodynamics, heat transfer, and geometry. The design utilizes the simplest geometry arrangements to date.